The invention relates to an aqueous colloidal suspension of the hydroxide of at least one transition element and/or of tin whose oxidation-reduction potential is less than that of the CrO42−/Cr(OH)3 couple (−0.12 V) with a pH between 2 and 11, limits excluded, and containing less than 0.02% of soluble ions of at least one transition element and/or of tin in solution, the suspension being designed to reduce the chromium VI content of cement to a value at most equal to 2 ppm.
The invention more particularly relates to an aqueous suspension of tin hydroxide with an oxidation-reduction potential of −0.96 V, of iron hydroxide with an oxidation-reduction potential of −0.56 V, of manganese hydroxide with an oxidation-reduction potential of −0.4 V, implemented alone or as a mixture, with a pH between 2 and 11, limits excluded, designed to reduce the chromium VI content of cement to a value at most equal to 2 ppm, the suspension being stabilised by means of a stabilisation agent.
The invention also relates to the use of aqueous suspensions of tin hydroxide, and/or iron hydroxide and/or manganese hydroxide of pH between 2 and 11, limits excluded, designed to reduce the chromium VI content of cement during the process of preparation of the cement to form a treated cement whose chromium VI content has a value at most equal to 2 ppm.
The invention finally relates to a process for treatment of cements to reduce the chromium VI content of the cement to a value at most equal to 2 ppm.
It is known that cements contain chromium compounds which, when the cements are mixed with water, appear in the form of chromium VI dissolved in water. Now, chromium VI soluble in the water of the cements may be the origin of allergic reactions for persons who come in contact with products containing hydrosoluble compounds of hexavalent chromium. Chromium is even suspected of being carcinogenic for the human. Thus, construction workers whose skin, in general the hands and the arms, is brought in regular contact with mixtures of cement and water are capable of contracting contact eczema due to the too-elevated chromium VI content of the cements.
Systems exist in the state of the art that are designed to reduce hydrosoluble hexavalent chromium compounds in the cements. It is known for example that the addition of ferrous sulphate reduces the content of chromium dissolved in a cement-water mixture. The ferrous sulphate can, for example, be added during the preparation of mixtures containing cement or during the manufacture of the cement. Iron (II) sulphate reduces Cr6+ to Cr3+, which has low solubility in the cement-water mixtures. Thus, the reaction between the Fe2+ and Cr6+ ions takes place in aqueous medium, i.e. when the water is added to the cement containing iron (II) sulphate.
Since the nineteen-seventies, cement manufacturers have been performing a ferrous sulphate treatment in particular at the time of the grinding step in order to reduce the chromium VI content in the cements and thus limit the risks of eczema being contracted by construction workers in regular contact with cement-water mixtures. However, this ferrous sulphate treatment of cements that makes it possible to reduce the chromium VI content of the treated cements turns out to be rather ineffective and presents numerous disadvantages.
As a matter of fact, it turns out to be necessary in practice to use iron sulphate in powder form in order to obtain reduction of chromium (VI) to chromium (III) with iron (II) ions. Now, oxygen in the air can convert iron (II) to iron (III). For this reason, particular attention as well as particular implementation conditions prove indispensable.
Other reduction pathways of chromium (VI) have been developed, as for example the reduction of chromium (VI) by organic systems such as aldehydes, and heterocyclic compounds such as, for example, pyridine. But use of these organic systems in practice has proven maladapted for economic reasons connected in particular to the low stability of these organic systems in the preparations of cement or to the quantities to be used. It is known, moreover, in the state of the art that a solution of soluble Sn2+ ions makes it possible to reduce chromium VI.
There is, therefore, in the state of the art, no aqueous suspension with a base of tin hydroxide, and/or iron hydroxide and/or manganese hydroxide of pH between 2 and 11, limits excluded, capable of reducing the chromium VI content of cement, cumulatively fulfilling the following conditions:
high stability, even in concentrated suspension,
viscosity of the suspensions constant over time and which remains low,
viscosity in concentrated solution sufficiently low so as to facilitate their use,
formulation at a pH between 2 and 11, limits excluded, to facilitate storage, transport and conditions of implementation.
Surprisingly, it was found that an aqueous suspension of the hydroxide of at least one transition element and/or of tin, in particular of tin hydroxide and/or iron hydroxide and/or manganese hydroxide with a pH between 2 and 11, limits excluded, reduces the chromium VI content of the cement to a value at most equal to 2 ppm and is characterised in that it comprises from 0.5 to 80% by weight of dry matter of the hydroxide of at least one transition element and/or of tin, in particular of tin hydroxide, and/or iron hydroxide and/or manganese hydroxide with respect to the quantity of water, the hydroxide being implemented alone or as a mixture and in that it is stabilised by a hydrosoluble stabilisation agent.
Consequently, this invention also has for object the use of these aqueous suspensions of tin hydroxide, and/or iron hydroxide and/or manganese hydroxide of pH between 2 and 11, limits excluded, designed to reduce the chromium VI content of the cement in order to produce cements whose chromium VI content is at most equal to 2 ppm, designated below as treated cements.
This invention also relates to a process for treatment of cements to reduce the chromium VI content of the cement to a value at most equal to 2 ppm, which is characterised in that, after the step of calcination of the clinker in the cement manufacturing process, an aqueous suspension is introduced, of tin hydroxide, and/or iron hydroxide and/or manganese hydroxide with a pH between 2 and 11, limits excluded, the suspension being as previously defined, according to the invention to fabricate cements whose chrome VI content is at most equal to 2 ppm, designated below as treated cements.
This invention also concerns the use of treated cement produced according to the treatment process of the invention and of which the chromium VI content is at most equal to 2 ppm in order to prepare a concrete composition comprising treated cement, water and the customary components.
The invention relates finally to compositions of concrete comprising cement, water and the customary components and being characterised in that, at the time of the mixing of the various components an aqueous suspension of tin hydroxide of pH between 2 and 11, limits excluded, is added, as defined within the framework of the invention in a quantity sufficient to reduce the soluble chromium VI content to a value at most equal to 2 mg of Cr(VI) per kg of cement.